Conventional LEDs comprise a semiconductor light generation region on a light absorbing substrate. Such LEDs enjoy various industrial applications, as in optical communication systems, optical information processing and as a light source due to their low power consumption, efficiency and reliability. Efficient operation of an LED requires uniform lateral spreading of current injected by a front electrical contact, so that the current uniformly enters the light generation region, thereby generating light with uniformity. However, as a result of current crowding, current tends to concentrate under the front electrical contact, thereby preventing uniform light generation. Industry efforts have focused upon reducing the current crowding problem as well as increasing the brightness of emitted light.
A traditional semiconductor LED is schematically illustrated in FIG. 1 and comprises a back electrical contact 10, an n-type substrate 20, a double heterostructure 30 (light generation region) which includes an undoped active layer 3b positioned between doped confinement layers 3a and 3c, and a front contact 70. It is in such a structure that current crowding typically occurs between the light generation region 30 and front contact 70, thereby preventing uniform light generation.
A prior effort to alleviate the current crowding effect and maximize light output is disclosed by Fletcher et al. in U.S. Pat. No. 5,233,204 and schematically illustrated in FIG. 2, wherein elements similar to those depicted in FIG. 1 bear similar reference numerals and, hence, are not described in detail to avoid repetition. The improvement disclosed by Fletcher et al. comprises positioning a relatively thick transparent semiconductor window layer 40, e.g., about 10 microns to about 50 microns, between the light generation region 30 and the front metal contact 70. Window layer 40 is desirably selected from materials having a high conductivity to enable rapid current spreading from front contact 70, thereby minimizing the current crowding effect. In addition, window layer 40 should have a higher bandgap than that of the light generation region 30 so that window layer 40 is transparent to emitted light. There are, however, drawbacks attendant upon the semiconductor LED illustrated in FIG. 2. For example, semiconductor window layer 40 can not include material systems having lattice constants which are not compatible with light generation region 30, thereby limiting design flexibility. In addition, the growth of a thick layer is time consuming.
Another prior approach to the current crowding problem is disclosed by Lin et al. in U.S. Pat. No. Re. 35,665 and schematically illustrated in FIG. 3, wherein elements similar to those in FIGS. 1 and 2 bear similar reference numerals. The semiconductor LED illustrated in FIG. 3 basically differs from that of FIG. 2 in that the thick semiconductor window layer 40 (FIG. 2) is replaced by transparent conductive oxide window layer 50 and an ohmic contact layer 51 which is typically a semiconductor material having a relatively high impurity concentration, e.g., greater than about 1.times.10.sup.18 atoms cm.sup.-3. Ohmic contact layer 51 is provided so that window layer 50 can be formed on a p-type confinement layer (3c), thereby expanding utility to n-type gallium-arsenide (GaAs) substrate-based LEDs. The transparent conductive oxide 50 comprises tin oxide, indium oxide, or indium-tin oxide, which are conductive materials, relatively inexpensive and relatively easier to grow than semiconductor compound transparent window materials for window layer 40 (FIG. 2).
With continued reference to FIG. 3, the utilization of a transparent conductive oxide layer 50 could reduce the current crowding effect, reduce manufacturing time, improve efficiency and expand applicability to LEDs with n-type GaAs substrates. Such oxides are suitable window materials for LEDs employing aluminum-gallium-indium-phosphorous (AlGaInP) material systems, i.e. for the light generation region, which emit light having wavelengths ranging from about 570 to about 680 nm. However, semiconductor LEDs based upon FIG. 3 are also problematic. For example, tin oxide, indium oxide and indium tin oxide exhibit poor optical transmission at longer wavelengths and, hence, are not particularly suitable for use in semiconductor LEDs at wavelengths of about 1.3 or about 1.5 .mu.m. Such oxides are also toxic, and do not exhibit long term chemical stability. In addition, semiconductor LEDs based upon FIG. 3 exhibit an undesirably high contact resistance between light transmission region 30 and ohmic contact layer 51, which unnecessarily squanders electricity and increases the operating temperature, e.g., above room temperature, thereby decreasing device reliability, i.e. longevity.
There exists a need for a semiconductor LED which exhibits improved light brightness, reduced crowding effect and increased longevity. There also exists a need for such a semiconductor LED which can be manufactured efficiently and economically.